Polyolefin microporous film and method for preparing the same

A method for producing the microporous polyolefin membrane having a weight-average molecular weight of 5×105 or more, and having a porosity 45 to 95%, tensile elongation of 300% or more, bubble point exceeding 980 Kpa and thermal shrinkage in the TD direction of 3% or less which comprises the steps of (a) preparing a solution comprising 10 to 50 wt. % of (A) an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5×105 or more, or (B) a composition containing an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 5×105 or more, and 90 to 50 wt. % of a solvent, (b) extruding the solution, (c) quenching the extruded solution to have a gel-like extrudate, (d) biaxially stretching the gel-like extrudate at 116 to 125° C. at a stretching ratio of 2 or more in both MD and TD directions, (e) removing the solvent and drying the extrudate, and (f) heat-setting the extrudate at 115 to 120° C.